I had a similar incident a few years back with my daughter who was about 9 at the time, whilst I went to the loo (crafty smoke tbh) some idiots started to pick on her because,despite her age & size, she had asked one to be quiet.
I returned to our seats to find her crying and very quickly was informed by my daughter what had happened.
I ejected the 3 cretins myself and got a round of applause from the audience (Odeon Festival park, Stoke)
On my way out, sure enough, they were waiting for me but I guess something about me told them that they really did not want to take it further.....
Pretty stupid tbh, I could have been stabbed in this day & age, that said, if you don't stand up to them, they win imo.
Sadly, my daughter never wanted to go back after that.